1. Field of the Invention
The present invention relates to power semiconductor modules used in power conversion devices for controlling driving current for electric equipment such as motors, and more particularly to the structure of an electric connection between the power semiconductor chip and the main circuit interconnection in the power semiconductor module.
2. Description of the Background Art
FIG. 8 is a sectional view showing the structure of the main part of a general-purpose IGBT (Insulated Gate Bipolar Transistor) module as an example of a conventional power semiconductor module. In the drawing, an insulating substrate 2 is formed on a radiation base board 1 made of Al (aluminum), Cu (copper), or the like. The insulating substrate 2 is made of alumina, AlN (aluminum nitride), or the like, and thin sheet of metal, such as Cu, is bonded on both sides thereof. The insulating substrate 2 is fixed on the radiation base board 1 with solder, for example. A power semiconductor chip 3, e.g. an IGBT, is provided on the insulating substrate 2. The IGBT shown in FIG. 8 has an emitter electrode 4 on its upper surface and a collector electrode 5 on its under surface, where the collector electrode 5 is electrically connected to the thin sheet of metal (not shown) on the insulating substrate 2 with a conductive material like solder. Busbars 6 and 7 are provided for the emitter and collector, which form the main part of the main circuit interconnection of the semiconductor module. The busbars 6 and 7 are electrically connected to relay substrates 8 and also to external interconnection outside the case (not shown). Like the insulating substrate 2, the relay substrates 8 are also insulating substrates having a thin sheet of metal, e.g. Cu, bonded on both sides thereof. The relay substrates 8 are fixed on the radiation base board 1 with solder, for example. The power semiconductor chip 3 and the surface of the relay substrates 8 are electrically connected through Al (aluminum) wire bonds 9. The case 10 is designed to accommodate the power semiconductor module, inside of which is molded with silicone gel 11.
In the conventional power semiconductor module, the power semiconductor chip and the busbars are electrically connected through the Al wire bonds as explained above.
For another method of making electric connections in the power semiconductor module, a method by mechanical contact is particularly adopted for large capacity devices, in which contact surfaces of conductors to be connected are pressurized with pressure applied from outside of the module.
The power semiconductor chip and the main circuit interconnection are electrically connected by wire bonding or mechanical contact in the conventional power semiconductor modules as stated above.
When the power semiconductor chip and the main circuit interconnection are electrically connected by using Al wire bonding in a conventional power semiconductor module, the wire-bonded connection do not suffer serious problems if the module has small capacity rated below 5A, for it will not generate much heat when operating. However, a module having larger capacity generates a great deal of heat in severe temperature cycle when it operates. Then thermal stresses are generated due to the difference in thermal expansion between the power semiconductor chip 3 and the Al wire bond 9 and cause the contact surface between the power semiconductor chip 3 and the wire bond 9 to peel off, which raises the problem that it cannot ensure reliability in long time use.
More specifically, the mode of the thermal stresses mentioned above is largely different from that of thermal stresses applied to common semiconductor chips. A common semiconductor chip itself generates less heat and thermal stresses are mostly produced by external factors such as solder reflow in the assembly process and heat shock cycle in the reliability evaluating test. On the other hand, the power semiconductor modules suffer from the thermal stresses generated by the frequent and sharp temperature cycles caused by a great deal of heat generated by the power semiconductor chips when operating (when a large current flows). This problem of course becomes more serious in modules having larger capacity. Large capacity modules like those used in the fields of electric railroad and steel manufacture must ensure long life of 20 to 30 years under such severe circumstances.
Accordingly, power semiconductor modules are required to clear heat shock cycle test generally called "power cycle test," in which large current is intermittently passed to a power semiconductor chip to repeatedly vary the temperature very rapidly in an enormous number of times.
The testing is described in RELIABILITY TESTING AND ANALYSIS OF IGBT POWER MODULES, written by Peter Yacob, Marcel Held, Paolo Scacco, Wuchen Wu, IEE Colloquium on "IGBT propulsion drives" Apr. 25, 1995. In the power cycle test, conventional power semiconductor modules are mostly broken in the mode of junction peeling at wire bond connection in 80,000 to 200,000 times of cycles under the condition of .DELTA.Tj=70.degree. C. Where .DELTA.Tj indicates temperature variation at the chip in one cycle.
Aside from the above-described problem of thermal stresses, the structure in which the chip and an electrode of the main circuit interconnection are joined by wire bonding in the power semiconductor module suffers the problems that the wire has small section and that only a limited number of wires can be bonded. Then the electrode (emitter electrode) formed on the surface of the chip cannot be effectively used and then the current shunt characteristic may be deteriorated. In a large-capacity module, the density of current flowing through the wire may become very high and then the over current will cause disconnection.
Further, for the process of manufacturing the power semiconductor modules, there is a trend toward application of larger pressurizing force to the wire-bonded junction surfaces to increase the junction strength of wires. Particularly, when the gate-emitter insulating film is formed under the emitter electrode surface as in the case of MOS semiconductor chip, this pressurizing force produces problems such as inferior insulation between the gate and emitter, breakage of the chip, etc., which leads to the problem of reducing the yield.
The conventional method of making electric connection through mechanical contact which has been adopted for large-capacity power semiconductor modules raises maintenance problems because variations in the pressurizing force largely influence the electric characteristics of the modules. Furthermore, when a mechanical-contact type power semiconductor module is used in a power conversion device such as an inverter, the device must be constructed as a large-scale device of stacked type, for example.
As explained so far, it is necessary for the power semiconductor modules to ensure electric connections which can allow stable passage of large current for long time at connections between materials having largely different linear expansion coefficients, such as between the power semiconductor chip and metal like copper or aluminum, under the sharp and frequent variations in temperature repeated by the heat generated by the power semiconductor chip itself. Particularly, with the recent development of power semiconductor modules toward larger capacity, more compact structure, higher switching speed, etc., the heat generation density of the power semiconductor chips is becoming very high, and ensuring the long-term reliability of the electric connections is the most important problem for the power semiconductor modules.